The present invention is directed to electric motors, and more particularly to a structure for an electric motor that makes the electric motor more immune to creating electro-magnetic interference (EMI).
The stator of an electric motor 12 is energized by a switching inverter 10 such as depicted in FIG. 1. The switching inverter 10 has very fast transitions, with a fast dv/dt applied to the stator windings. The electric motor 12 has multiple capacitive paths. Two of these capacitive paths are of primary concern. They include the path between the stator windings and the motor frame, Cwf, (ground) capacitance (14), and the path between the stator windings and the rotor (17), Cwr, capacitance (16). A very rapid dv/dt is applied across each of these capacitances, Cwf, Cwr, causing an electrical current to flow through these capacitive paths when the inverter 10 switches.
Current flowing in these capacitive paths causes two major problems. These problems are associated with EMI as well as excessive bearing currents. If for example, the inverter 10 is connected to a 250 VDC link, and the semiconductor switches turn on and off in 50 nsec, the dv/dt will be 5×109 V/sec. Typical stator winding to ground capacitance is about one (1) to about ten (10) nF. Thus, assuming 2 nF of capacitance, 10 A of peak current will be flowing through this path. This is a substantial amount of common-mode current; and it requires very large, expensive, heavy common-mode filters to attenuate this current. Current that flows through the Cwr path will flow to ground through the rotor bearings. This current can cause degradation of the bearings.
In view of the foregoing, it would be advantageous and beneficial to provide a motor structure that reduces EMI problems as well as common-mode currents that flow into the rotor bearings and electrical ground structures generally associated with conventional electric motor structures.